Evaporative Light Scattering Detectors (ELSD) are commonly used in high performance liquid chromatography (HPLC) or supercritical fluid chromatography (SFC) because they are capable of detecting a wider variety of analytes than many other types of chromatographic detectors. Prior ELSD's comprise a nebulizer which receives the eluent from the chromatograph and generates an aerosol comprising droplets of the mobile phase. When an analyte elutes from the chromatograph these droplets will also comprise dissolved or suspended particles of the analyte. The aerosol generated by the nebulizer is passed into a heated desolvation region wherein the mobile phase evaporates leaving dry particles of the analyte. The particles then pass through a light beam and their presence is detected by measuring the scattered light from the beam.
The simplest prior ELSD's comprise a nebulizer, desolvation region and a light scattering region. An example of such a prior ELSD is given in U.S. Pat. No. 6,229,605 B1. Typically, the nebulizer is either a co-axial or cross-flow pneumatic nebulizer which uses a flow of inert gas to produce an aerosol from the eluent from the liquid chromatograph. This aerosol is generated directly in the desolvation region, which may comprise a heated drift tube, which solvent is evaporated from the aerosol. At the exit of the drift tube, desolvated analyte particles from the aerosol pass through a light beam, typically disposed perpendicularly to the direction of travel of the desolvated particles. Light scattered from the beam by the particles is detected by one or more detectors, (typically photomultipliers) disposed so that neither the particles themselves or the laser beam strike them. The signal from the detector(s) is amplified and fed to a display device such as a chart recorder or a computer for further processing.
The signal from the detector(s) is a measure of the quantity and size of analyte particles entering the laser beam, and hence a measure of the concentration of the analyte in the chromatograph eluent. As explained, an ELSD will produce a signal from almost all analytes providing that they are sufficiently involatile to avoid loss by evaporation in the drift tube. However, many factors affect the response of the ELSD, including the size and shape of the analyte particles, their chemical and physical properties, the nature and flow rate of the mobile phase, and the parameters of the nebulizer and drift tube, for example, nebulization gas flow and drift tube temperature. It is necessary to adjust these latter parameters carefully in order to obtain optimum performance, and the optimum values frequently depend on the mobile phase flow rate and composition, which can cause problems when gradient elution is employed.
It has been found that best results are obtained when the nebulizer generates an aerosol of relatively uniform droplet size. If it does not, large droplets may not be completely evaporated in the drift tube and may pass into the laser beam, generating signals even when no analyte is present in them. Increasing the drift tube temperature can help to evaporate those larger droplets, but risks evaporation of relatively volatile analytes because once the solvent is completely evaporated the temperature of an analyte particle may rise rapidly.
To mitigate the problem of droplet size, many ELSD's comprise a separate nebulizer chamber between the nebulizer itself and the heated drift tube.
The nebulizer chamber is typically unheated, so that the larger droplets in the aerosol separate out by condensation on the walls. A drain is provided to remove the condensed liquid. In some cases an impactor is provided, on which larger droplets in the aerosol will impinge and be lost, while smaller droplets are carried around it by the gas flow through the nebulizer chamber.
The average size of the droplets entering the heated drift tube is therefore smaller than it would be if the nebulizer chamber was not provided, which allows a lower drift tube temperature to be used while still completely desolvating the analyte particles. This in turn reduces analyte losses by evaporation. Use of a separate nebulizer chamber also facilitates operation with a supercritical fluid chromatograph, for example one using compressed carbon dioxide as a mobile phase.
A disadvantage of the additional chamber is, however, the loss of analyte that may be present in the larger droplets that condense in it, reducing sensitivity. Nevertheless, the majority of currently available ELSD's incorporate a separate nebulizer chamber. An example of such a commercially available ELSD is the Waters model 2420, (Waters Corporation, 34 Maple Street, Milford, Mass. 01757, USA). Other prior ELSD's comprising separate nebulizer chambers are described in WO 2004/077047, U.S. Pat. No. 6,362,880 and U.S. 2003/0086092 A1.
A typical application for an ELSD is for the HPLC analysis of complex mixtures of natural products. In these applications, the chromatographic separation may typically take between 20 and 120 minutes and large numbers of unknown compounds may be present. In such applications, the use of a UV-absorbance detector is risky because some of the unknown compounds may fail to be detected even when present in large concentrations, because they have no UV-chromophore. An ELSD has a more universal response and hence is more suitable in this application. Recently, ELSD's have begun to be used by chemists involved in the sythesis of new drug candidates by combinatorial chemistry. In this application, the HPLC analysis frequently last only 2 or 3 minutes. Unfortunately, prior ELSD's typically require as long as 15-20 minutes to stabilize at the commencement of an analysis, which increases the total analysis time by up to a factor of 10 and renders the use of the ELSD less cost effective in comparison with other detectors, despite its other advantages.